1. Field of the Invention
The invention relates to the field of apparatus for loading nuclear fuel rods into skeleton assemblies, and more particularly to a loading apparatus having an array of automatically selectable pull rods which are advanced and retracted by a linear actuator for pulling selected fuel rods from a supply magazine into the skeleton. A processor controls the pull rod selection for patterned loading and/or for leaving empty thimble tube spaces in the array according to a stored pattern. The pull rods encompass a line in the array and the processor controls carriage position for advancing from line to line, and push/pull rates, for effecting programmed sequences.
2. Description of the Prior Art
Nuclear reactors of the type used to generate power by producing heat in fissile material generally have reactor cores comprising numerous fuel rods arranged in spaced-apart arrays in supporting skeletons, each of which can be handled as a unit. The skeletons have a top plate and a bottom plate known as nozzles, and a number of intermediate plates known as grids. The grids have openings through which the fuel rods extend and are held parallel to one another in a laterally spaced array whereby water or other coolant can be circulated over the fuel rods to extract heat for useful generation of power. Typically the grids form spring structures which bear inwardly on each of the rods to hold the rod securely notwithstanding the flow of coolant.
The grids and the nozzles are fixed relative to one another by thimble tubes which attach respectively to the grids and nozzles to form a structural unit for supporting the fuel rods. The thimble tubes occupy positions in the array which would otherwise have fuel rods, and form guides for control rods made of neutron absorbing material which can be inserted into the skeleton via the thimble tubes to regulate neutron flux and thereby to regulate the heat generated by the reactor core. Additionally, the skeleton typically has a further tube which carries instrumentation, and is also attached at the grids and nozzles.
Each fuel rod is a thin elongated tube which carries a stack of cylindrical fuel pellets, e.g., of uranium dioxide. The fuel rods may be, for example, 4 meters in length and only one centimeter in diameter, and thus must be supported and handled carefully to avoid damage. A rupture in a fuel rod could release radioactive fuel into the coolant. All the fuel pellets in a given fuel rod are usually of the same enrichment. However, this can be varied and the skeletons can be arranged such that the enrichment of the fuel in the rods is different for different positions in the array of rods. For example, boiling water reactors have fuel assemblies with fuel rods at different enrichments arranged in a planned pattern.
It is known to load and unload fuel rods in skeleton assemblies with the assistance of motorized means arranged to move a grasping apparatus through the fuel rod openings in skeleton grids. The fuel rods are provided with end plugs defining shapes which can be engaged or released. In U.S. Pat. No. 4,747,997--Boatwright, a fuel rod grasping apparatus is disclosed for this purpose. The grasping apparatus comprises a hollow tube terminating in an engagement fitting having a contoured shape with axial slots. A movable plug is advanced through the tube to expand the engagement fitting, thus securing the grasping apparatus in a complementary contoured shape in the end plug of a fuel rod. When the movable plug is retracted, the engagement fitting compresses resiliently and releases the fuel rod. The grasping apparatus can be coupled to actuators for advancing or retracting the rods from the skeleton assembly.
For loading a skeleton with fuel rods, the grasping apparatus is advanced through the supporting grids of the skeleton, engaged with a fuel rod in a magazine, then retracted to pull the fuel rod into position in the skeleton. The grasping apparatus must overcome the frictional engagement of the spring structures of the grids against the fuel rod while the rod is pulled into position.
In the reactor core the fuel rods are normally positioned vertically in the skeleton. When pulling fuel rods from the magazine into the skeleton, it is convenient to position the magazine and the skeleton such that the fuel rods are horizontal. The skeleton can be clamped in position relative to the grasping apparatus, for example as disclosed in U.S. Pat. No. 4,760,637--Kerrey et al.
U.S. Pat. No. 4,894,848--Lambert et al discloses the use of a programmed processor for assisting in patterned loading of fuel rods into skeletons from a supply of fuel rods of different enrichments. The loader is disposed between the skeleton and a supply of fuel rods, and pushes rods one-by-one from the supply into the skeleton rather than reaching through the skeleton to engage and pull the rods back into the skeleton. The processor is coupled to means for selecting a particular bin in the supply, means for positioning the fuel assembly skeleton vertically and horizontally for selection of rod location, and data input means. The processor maintains a database of selectable enrichment patterns for skeletons and also maintains a database of codes defining the enrichment of the fuel rods stored in the supply bins, corresponding to bar code labels on the fuel rods. The supply bins, feeding apparatus and skeleton are all mounted parallel to a feed path defined by motorized pinch rollers. One of the fuel rods is advanced manually from the supply bin to a feed position where a bar code reader detects the bar code on the rod. The processor determines the enrichment of the rod from the bar code, and positions the fuel assembly skeleton to align one of the positions in the X-Y array of fuel rods to the feed path, as appropriate to the particular enrichment pattern selected. The fuel rod is then advanced into the skeleton until it clears the pinch rollers. A supplemental pushing apparatus completes the advance of the fuel rod into position.
The fuel rod positions in the skeleton assembly typically are geometrically regular, except spaces that are left for the control rod thimbles and instrumentation tubes. These positions are programmed in the programmed pattern such that the loader passes over them when installing the rods.
It is important to handle the fuel rods carefully while passing the rods through the springs engaging the rods at the grids. Obstructions which are encountered, particularly when pushing rods but also when pulling them inaccurately or at the wrong speed, can cock dimples in the grids or cause other loading damage. Loading damage may be difficult to detect, especially if the rods are loaded in a sequence in which rods loaded initially obstruct the operator's view of rods loaded later. Damage to the rods increases the possibility that the rods will fail, and perhaps release radioactive material into the primary reactor coolant.